1. Field of the Invention
The present invention relates to an automatic reduction-ratio changing apparatus which makes it possible to automatically change the speed reduction ratio transmitted from an output shaft to a displaceable member of an actuator by using a planetary gear mechanism, an internal gear lock mechanism, and an internal gear lock release mechanism.
2. Description of the Related Art
A nut runner, which is an automatic tightening tool for screws of, for example, nuts and bolts, is known as an apparatus to which a speed reduction ratio changing mechanism is applied.
The nut runner of this type comprises a sun gear and planet gears. The sun gear and the planet gears, which function as a first speed reduction section and which constitute a planetary gear mechanism, are provided at an end of a drive shaft of an air motor. A first output shaft, which is integrally connected to the planet gears of the first speed reduction section, has a decreased number of revolutions so that the output torque thereof is large. The nut runner further comprises second and third speed reduction sections each of which comprises a planetary gear mechanism as well. The second and third speed reduction sections are connected to the first speed reduction section so that the speed is further reduced. Accordingly, a high torque is generated on a nut-tightening socket which is installed to an output shaft of the third speed reduction section. Thus, the nut is tightened by means of the output torque.
For example, Japanese Laid-Open Patent Publication No. 9-295278, which relates to a nut runner as described above, discloses the following system. That is, an internal gear, which is included in a planetary gear mechanism as a speed reduction section, is rotatable. Further, a clutch section is arranged for the internal gear, in which an output shaft and the internal gear are rotated in an integrated manner when no load is applied to the output shaft. When some load is applied to the output shaft, the rotation of the internal gear is stopped, and the output shaft is rotated at a reduced speed by the clutch section.
However, if the technical concept disclosed in Japanese Laid-Open Patent Publication No. 9-295278 is applied, for example, to an actuator which reciprocates such as a piston rod of a fluid pressure cylinder and a slider of a linear actuator, the speed reduction ratio cannot be switched or changed even when an air motor is reversed, because it is impossible to disengage the bitten roller. In this situation, the piston rod or the slider, which has been displaced along the outward or outgoing route, cannot be displaced thereafter at a high speed along the homeward or return route.
In other words, the following reason may be pointed out. In the case of the technical concept disclosed in Japanese Laid-Open Patent Publication No. 9-295278, after the rotation of the internal gear is stopped and the high torque is generated at the terminal end of the outward route, the displacement is performed at a low speed along the homeward route while retaining the state in which the rotation of the internal gear is stopped, without switching the speed reduction ratio.